The electrical power system in the United States generates three-phase alternating current (AC) electrical power. Each power phase is 120 degrees out of phase, plus or minus, with the other two power phases. The voltage of any phase oscillates sinusoidally between positive voltage and negative voltage. It happens that three-phase power distribution provides an acceptable compromise between electrical generation and distribution efficiency and the expense and complexity of power distribution equipment.
It is more efficient to transmit electrical power at high voltage levels than at low voltage levels. Electrical power is generated as three-phase AC power at moderate voltage levels in the 12 thousand volt (kV) to 25 kV range. The voltage level is stepped up to the 110 kV to 1000 kV range using a transformer for transmission over long transmission lines, hence minimizing transmission line power loss. The transmission line voltage is stepped down, using a transformer at a substation, to the 12 kV to 35 kV range for local distribution. The local distribution voltage level is further stepped down through one or more transformer stages to provide 120 volt AC power to the home and office. Special accommodations may be made for manufacturing plant electrical power consumers.
Switchgear is employed to control the transmission systems. The switchgear may include circuit breakers, fuses, switches, and relays. Electrical power distribution protective relays, hereinafter referred to simply as “relays,” monitor a variety of electric power distribution parameters and control circuit breakers based on the state or condition of the electric power distribution parameters. For example, if too much current is being provided to a local distribution line, a relay may command a circuit breaker or switch gear to open, thus interrupting the supply of electrical power to that local distribution line. Current, voltage level, frequency, phase, and other parameters may be monitored by relays.
Relays vary in complexity from electromechanical devices monitoring a single parameter to microprocessor controlled relays capable of monitoring many independent parameters concurrently. The relays are capable of changing their behavior when reprogrammed for monitoring various characteristics of a power distribution system. For safe and reliable operation of the power distribution system it is necessary to thoroughly test relays in the manufacturer's plant, prior and upon installation in the power distribution system, and at periodic intervals after installation. Failures of the power distribution system, such as the power blackout in the northeastern United States and southeastern Canada in 2003, can result in significant economic losses and inconvenience, and perhaps danger to those requiring special medical services dependent on electrical power distribution.
Testing of relays may be accomplished by emulating theoretical fault characteristics of a power distribution or generation system, thereby validating design elements associated with protection systems on a power distribution or generation system. This emulation consists of various parameter characteristics associated with the power distribution system during a fault. Fault conditions, generated through emulation, are therefore applied to the relay, and expected results are anticipated, validated or settings corrected for correct operation when the relay is applied in an actual power distribution system.